Electrodeposition of silver with fluoropolymer nanoparticles

ABSTRACT

Electrolytic plating compositions and electrolytic plating processes for the co-deposition of silver or silver alloy with fluoropolymer nanoparticles are provided. The silver or silver alloy composite coating containing fluoropolymer nanoparticles has enhanced functional properties such as a reduced coefficient of friction. The electrolytic plating composition comprises: (a) a silver ion source comprising silver methane sulfonate (Ag-MSA); (b) a complexing agent comprising a compound comprising a nitrogen-containing heterocyclic ring; (c) a pre-mix dispersion comprising fluoropolymer nanoparticles particles having a mean particle size of from about 10 nm and about 500 nm and a surfactant; and (d) an auxiliary surfactant comprising a cationic fluorosurfactant, wherein the composition has a pH of from about 8 to about 14.

FIELD OF THE INVENTION

The present invention generally relates to electrolytic plating compositions and electrolytic plating processes for the co-deposition of silver or silver alloy with fluoropolymer nanoparticles to provide a silver or silver alloy composite coating with enhanced functional properties.

BACKGROUND OF THE INVENTION

In general, pure silver coatings are highly conductive and are harder than gold. For these reasons, these coatings would be especially suited for electrical connectors. However, pure silver coatings have a high coefficient of friction (COF). As a result, high insertion forces are required for electrical connectors coated with silver. Also, these coatings have poor wear characteristics (poor durability). The severity of wear of the coating generally increases with increased normal forces. Consequently, these characteristics limit the application of pure silver coatings in connector applications.

Coatings made from fluoropolymers such as polytetrafluoroethylene (PTFE) are known hydrophobic materials. Since a hydrophobic surface is water repellent and inhibits the adsorption of environmental moisture, fluoropolymer coatings can be used to impart corrosion resistance to a metal surface.

Some methods of electrolytically depositing silver-based composite coatings comprising fluoropolymer nanoparticles are known. For example, U.S. Patent Application Publication 2010/0294669 (Abys et al.) describes a method for enhancing surface properties including water repellency, corrosion resistance, hardness, wear resistance, and lubricity by electrolytically depositing a metal-based (e.g., a silver-based) composite coating containing fluoropolymer nanoparticles. The electrolytic plating solution of Abys et al. includes inorganic silver salts such as silver nitrate or silver oxide and a dispersion of fluoropolymer nanoparticles coated with surfactant molecules. However, the silver plating solutions disclosed by Abys et al. provide limited stability for the fluoropolymer particles and, in some instances, provide coatings having an undesirable appearance.

There remains a need for an electrolytic composition and process that yields a silver-based composite coating having a smooth, lubricious, and corrosion resistant surface that is highly suited for applications such as connector coatings which require reduced insertion force to decrease wear. Further, there remains a need for an electrolytic composition and process that is highly stable and provides uniform deposits with good appearance over the life of the bath.

SUMMARY OF THE INVENTION

Generally, the present invention is directed to electrolytic plating compositions and processes for depositing a composite silver or silver alloy coating on a substrate. In one aspect, an electrolytic plating composition according to the present invention comprises

(a) a silver ion source comprising silver methane sulfonate (Ag-MSA);

(b) a complexing agent comprising a compound comprising a nitrogen-containing heterocyclic ring;

(c) a pre-mix dispersion comprising fluoropolymer nanoparticles particles having a mean particle size of from about 10 nm and about 500 nm and a surfactant; and

(d) an auxiliary surfactant, wherein the composition has a pH of from about 8 to about 14.

In another aspect, a process for applying a silver or silver alloy composite coating onto a substrate surface according to the present invention comprises contacting the substrate surface with the electrolytic plating composition and applying an external source of electrons to the electrolytic plating composition to thereby electrolytically deposit the composite coating onto the substrate surface, wherein the composite coating comprises silver or silver alloy and the fluoropolymer nanoparticles.

Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B present profile plots of coating surfaces prepared without PTFE particles and with PTFE particles, respectively, and after a wear test applying 50 g load over the course of 500 sliding cycles.

FIG. 2 presents a plot of the aging time vs. rate of PTFE co-deposition for compositions containing selected surfactants.

FIG. 3 presents a series of images of coatings prepared with plating compositions containing varied concentrations of PTFE and after two different thermal aging tests.

FIG. 4 presents a series of scanning electron microscope (SEM) images of freshly coatings prepared with plating compositions containing varied concentrations of PTFE.

FIG. 5 provides a SEM image of a composite silver coating prepared from a bath containing 40 g/I PTFE.

FIG. 6 shows SEM images of the coated surface grain structures after 168 hours of heating at 125° C.

FIG. 7 shows SEM images of the coated surface grain structures after 100 hours of heating at 150° C.

FIG. 8 shows focused ion beam (FIB) images of a cross-section of selected samples after 100 hours of heating at 150° C.

FIG. 9 shows focused ion FIB images of a cross-section of selected samples after 100 hours of heating at 150° C.

FIG. 10A presents a contact resistance measurement plot for freshly coated substrates prepared in Example 5 (no aging).

FIG. 10B presents a magnified selection of the FIG. 10A plot between a contact resistance of 1 and 2 milliohms.

FIG. 10C presents a contact resistance measurement plot for coated substrates prepared in Example 5 after 100 hours of heating at 150° C.

FIG. 10D presents a contact resistance measurement plot for coated substrates prepared in Example 5 after 168 hours of heating at 125° C.

FIG. 10E presents a magnified selection of the FIG. 10D plot between a contact resistance of 1 and 4 milliohms.

FIG. 11A presents the results of an AES analysis as a function of coating depth for a pure silver coating on a copper substrate.

FIG. 11B presents the results of an AES analysis as a function of coating depth for a composite coating deposited from a plating composition containing 10 g/L of PTFE.

FIG. 12 presents images of substrates coated in accordance with Example 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, the present invention is directed to electrolytic plating compositions and electrolytic plating processes for the co-deposition of silver or silver alloy with fluoropolymer nanoparticles to provide a silver or silver alloy composite coating with enhanced functional properties.

One aspect of the present invention provides an electrolytic plating composition and process that provide a silver-based composite coating having a reduced coefficient of friction. A coating having a reduced coefficient of friction is especially useful for an electrical connector surface that is subjected to insertion forces. A connector surface having a reduced coefficient of friction decreases insertion forces and consequently decreases wear on the connector surface.

Another aspect of the present invention provides an electrolytic plating composition and process for depositing a silver-based composite coating which provides improved corrosion resistance. When the silver-based composite coating is deposited on a corrodible surface such as copper or copper alloy, the silver-based composite coating provides a barrier that limits contact of the copper surface with oxygen and restricts diffusion of copper to the surface of the silver-based coating, which prevents formation of copper oxide on the surface of the coating. A silver-based composite coating that provides improved corrosion resistance is especially suitable for electrical contact finish.

Yet another aspect of the present invention provides an electrolytic plating composition and process for depositing a silver-based composite coating that provides improved contact resistance stability even after thermal aging. Contact resistance is the electrical resistance associated with the interface between electrical connections. Thermal aging of copper and copper alloy surfaces is known to negatively increase the contact resistance of the surface, which can lead to possible malfunction of the electrical connections. A silver-based composite coating that maintains a good contact resistance after thermal aging is highly desirable for electrical connectors without having to use a separate nickel barrier layer between the silver-based coating and copper-based substrate.

Another aspect of the present invention provides a stable electrolytic plating composition and process for depositing uniform silver-based composite coating over the course of the plating bath life. A stable electrolytic plating composition is essential to a cost-effective plating operation.

In accordance with the present invention, the electrolytic plating composition for depositing a silver-based composite coating on a substrate generally comprises (a) a source of silver ions comprising a silver salt of a sulfonic acid, (b) a complexing agent, (c) a pre-mix dispersion comprising fluoropolymer nanoparticles and a surfactant, and (d) an auxiliary surfactant. Further in accordance with the present invention, the process for depositing the silver-based composite coating comprising silver and fluoropolymer nanoparticles on a substrate comprises contacting the surface with an electrolytic plating composition and applying an external source of electrons to the electrolytic plating composition to thereby electrolytically deposit the composite coating onto the metal surface.

The electrolytic plating composition includes as a silver ion source at least one silver salt of a sulfonic acid such as silver methanesulfonate (Ag-MSA). Silver methanesulfonate has been found to be an especially stable and reliable source of silver ion in alkaline electrolytic plating solutions. Accordingly, in various embodiments, the silver ion source comprises silver methanesulfonate. In certain embodiments, silver methanesulfonate is the predominant or sole source of silver ion. Typically, the electrolytic plating composition according to the invention has a silver ion concentration from about 5 g/L to about 300 g/L, from about 10 g/L to about 200 g/L, from about 15 g/L to about 200 g/L, from about 10 g/L to about 100 g/L, from about 5 g/L to about 50 g/L, from about 10 g/L to about 50 g/L, or from about 20 g/L to about 40 g/L (e.g., about 30 or about 40 g/L).

In some embodiments, the electrolytic plating solution may contain additional silver ion sources including inorganic silver salts selected from the group consisting of silver oxide, silver nitrate, and silver sulfate. In these embodiments, the weight ratio of sulfonic acid silver salt to inorganic silver salt is from about 5:1 to about 1:5, from about 3:1 to about 1:3, or from about 2:1 to about 1:1.

For the deposition of silver alloy layers the electrolytic plating composition can include various sources of alloying metal ions such as gold, platinum, bismuth, and copper. Preferably, these metals are employed in the form of their sulfonic acid salts, oxides, nitrates, or sulfates.

The electrolytic plating composition of the present invention comprises a complexing agent. In accordance with various aspects, the electrolytic plating composition is cyanide-free. Accordingly, in various embodiments, the complexing agent comprises a compound comprising a nitrogen-containing heterocyclic ring. In particular, the compound comprising the nitrogen-containing heterocyclic ring comprises at least one 5-membered or 6-membered ring. Examples of nitrogen-containing heterocyclic rings include hydantoin, succinimide, pyridine, bipyridine, pyrimidine, uracil, substituted and unsubstituted analogs thereof, derivatives thereof, and combinations thereof. In some embodiments, the compound comprising the nitrogen-containing heterocyclic ring is selected from the group consisting of substituted and unsubstituted hydantoin and substituted and unsubstituted succinimide. In other embodiments, the complexing agent comprises succinimide.

In various embodiments, the complexing agent comprises a hydantoin compound of formula I

wherein R¹, R², R³, and R⁴ are independently hydrogen, an alkyl group having 1 to 5 carbon atoms, a hydroxyalkyl group having 1 to 5 carbon atoms, or a substituted or unsubstituted aryl group. In various embodiments, R¹, R², R³, and R⁴ are independently hydrogen or an alkyl group having 1 to 5 carbon atoms (e.g., methyl or ethyl). In these and other embodiments, R¹ and R² are each an alkyl group having 1 to 5 carbon atoms (e.g., methyl or ethyl) and R³ and R⁴ are each hydrogen.

In some embodiments, formula I is a compound selected from the group consisting of hydantoin; 1-methylhydantoin; 1,3-dimethylhydantoin; 5,5-dimethylhydantoin; 1-hydroxymethyl-5,5-dimethylhydantoin; 5,5-diphenylhydantoin; and mixtures thereof. In certain embodiments, formula I is 5,5-dimethylhydantoin.

Typically, the electrolytic plating composition contains at least about 50 g/L, at least about 60 g/L, at least about 70 g/L, or at least about 80 g/L of the complexing agent (e.g., a hydantoin compound of the formula I). In various embodiments, the electrolytic plating composition contains from about 50 g/L to about 300 g/L, from about 60 g/L to about 280 g/L, from about 70 g/L to about 250 g/L, from about 80 g/L to about 250 g/L, or from about 80 g/L to about 150 g/L of the complexing agent.

In accordance with the present invention, the electrolytic plating composition comprises fluoropolymer nanoparticles. The enhancement of some functional surface properties of the silver-based composite coating is at least in part due to co-deposition of the silver or silver alloy with fluoropolymer nanoparticles. By incorporating fluoropolymer nanoparticles having a mean particle size smaller than the wavelengths of visible light into the silver-based composite coatings of the present invention, enhanced functional surface properties such as water repellency, corrosion resistance, wear resistance, and lubricity are obtained without impacting the appearance of the coatings. In other words, an electrolytic deposition method that yields a bright, glossy coating without fluoropolymer nanoparticles yields a bright, glossy coating with fluoropolymer nanoparticles.

The fluoropolymer particles may be selected from among polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene copolymer (FEP), perfluoroalkoxy resin (PFE, a copolymer of tetrafluoroethylene and perfluorovinylethers), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chloro-trifluoroethylene copolymer (ECTFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF) and combinations thereof. In various embodiments, the fluoropolymer nanoparticles comprise PTFE nanoparticles.

The mean particle size of the fluoropolymer nanoparticles is preferably on the order of, or substantially smaller than, the wavelength of visible light (i.e., less than 380 nm to 780 nm). The mean particle size may be less than about 500 nm, less than about 250 nm, less than about 200 nm, less than about 150 nm, or less than about 100 nm. The mean particle size may be greater than 5 nm, greater than about 10 nm, or greater than about 50 nm. Accordingly, the mean particle size may be from about 5 nm to about 500 nm, from about 10 nm to about 500 nm, from about 10 nm to about 200 nm, or from about 50 nm to about 150 nm. In some embodiments, the fluoropolymer nanoparticles have a mean particle size from about 50 nm to about 100 nm. In other embodiments, the fluoropolymer nanoparticles have a mean particle size from about 10 nm to about 50 nm.

The mean particle sizes stated above refer to the arithmetic mean of the diameter within a population of fluoropolymer nanoparticles. A population of fluoropolymer nanoparticles contains a variation of diameters. Therefore, the particles sizes may be additionally described in terms of a particle size distribution, i.e., a minimum volume percentage of particles having a diameter below a certain limit. In various embodiments, therefore, at least about 50 volume % of the fluoropolymer nanoparticles have a particle size less than 200 nm, at least about 70 volume % of the particles have a particle size less than 200 nm, at least about 80 volume % of the particles have a particle size less than 200 nm, or at least about 90 volume % of the particles have a particle size less than 200 nm.

In some embodiments, at least about 30 volume % of the fluoropolymer nanoparticles have a particle size less than 100 nm, at least about 40 volume % of the particles have a particle size less than 100 nm, at least about 50 volume % of the particles have a particle size less than 100 nm, or at least about 60 volume % of the particles have a particle size less than 100 nm.

In further embodiments, at least about 25 volume % of the fluoropolymer nanoparticles have a particle size less than 90 nm, at least about 35 volume % of the particles have a particle size less than 90 nm, at least about 45 volume % of the particles have a particle size less than 90 nm, or at least about 55 volume % of the particles have a particle size less than 90 nm.

In other embodiments, at least about 20 volume % of the fluoropolymer nanoparticles have a particle size less than 80 nm, at least about 30 volume % of the particles have a particle size less than 80 nm, at least about 40 volume % of the particles have a particle size less than 80 nm, or at least about 50 volume % of the particles have a particle size less than 80 nm.

In still further embodiments, at least about 10 volume % of the fluoropolymer nanoparticles have a particle size less than 70 nm, at least about 20 volume % of the particles have a particle size less than 70 nm, at least about 30 volume % of the particles have a particle size less than 70 nm, or at least about 35 volume % of the particles have a particle size less than 70 nm.

The fluoropolymer nanoparticles employed in the present invention have a so-called “specific surface area” which refers to the total surface area per unit mass of nanoparticles. As particle size decreases, the specific surface area of a given mass of particles increases. Accordingly, smaller particles as a general proposition provide higher specific surface areas. The relative activity of a particle to achieve a particular function is in part a function of the particle's surface area in the same manner that a sponge with an abundance of exposed surface area has enhanced absorbance in comparison to an object with a smooth exterior.

In various embodiments, the invention employs fluoropolymer nanoparticles where at least about 50 wt. %, preferably at least about 90 wt. %, of the nanoparticles have a specific surface area of at least about 15 m²/g (e.g., between about 15 and 35 m²/g). The specific surface area of the fluoropolymer nanoparticles may be as high as about 50 m²/g. For example, a dispersion of DRYFILM WD-4560 (PTFE dispersion) has a specific surface area of about 23 m²/g. The nanoparticles employed in these embodiments may have a relatively high surface-area-to-volume ratio. These nanoparticles have a relatively high percent of surface atoms per number of atoms in a particle. For example, a smaller particle having only 13 atoms has about 92% of its atoms on the surface. In contrast, a larger particle having 1415 total atoms has only 35% of its atoms on the surface. A high percentage of atoms on the surface of the particle relates to high particle surface energy, and greatly impacts properties and reactivity.

Nanoparticles having relatively high specific surface area and high surface-area-to-volume ratios are advantageous since a relatively smaller proportion of fluoropolymer particles may be incorporated into the composite coating compared to larger particles, which require more particles to achieve the same surface area, and still achieve the effects of increased corrosion resistance and decreased coefficient of friction. Accordingly, as little as 10 wt. % fluoropolymer nanoparticles in the composite coating achieves the desired effects, and in some embodiments, the fluoropolymer nanoparticles component is as little as 5 wt. %, such as between about 1 wt. % and about 5 wt. %.

In the electrolytic plating composition, the fluoropolymer nanoparticles are typically present in a concentration that is at least about 1 g/L, at least about 2 g/L or at least about 5 g/L. In various embodiments, the concentration of fluoropolymer nanoparticles (e.g., PTFE) is between about 1 g/L and about 400 g/L, between about 1 g/L and about 200 g/L, between about 1 g/L and about 50 g/L, between about 1 g/L and about 40 g/L, between about 1.5 g/L and about 400 g/L, between about 2 g/L and about 200 g/L, between about 2 g/L and about 100 g/L, between about 2.5 g/L and about 50 g/L, between about 2 g/L and about 5 g/L, between about 5 g/L and about 200 g/L, between about 5 g/L and about 100 g/L, between about 5 g/L and about 50 g/L, between about 10 g/L and about 110 g/L, or between about 10 g/L and about 50 g/L.

If the nanoparticle source is TEFLON PTFE 30, for example, the concentration in the electrolytic plating composition may be achieved by adding between about 1.5 g and about 350 g, between about 5 g and about 170 g, or between about 10 g and about 100 g of 60 wt. % PTFE dispersion per 1 L of electrolytic plating composition. If the nanoparticle source is a dispersion containing approximately 50 wt. % PTFE, such as DRYFILM WD-4560 (48 wt. %), then the concentration in the electrolytic plating composition may be achieved by adding as little as about 1 g/L. In various embodiments, the PTFE concentration in the plating composition is between about 1 g/L and about 400 g/L, between about 1.5 g/L and about 400 g/L, between about 2 g/L and about 200 g/L, between about 5 g/L and about 200 g/L, between about 5 g/L and about 100 g/L, between about 5 g/L and about 50 g/L, between about 10 g/L and about 110 g/L, or between about 10 g/L and about 50 g/L. In volume terms, the concentrations in the electrolytic plating composition may be achieved by adding PTFE dispersion to the solution at a volume of between about 0.5 mL and about 160 mL of PTFE dispersion per 1 L of electrolytic plating composition, more preferably between about 6 mL and about 80 mL of PTFE dispersion per 1 L of electrolytic plating composition.

The high surface activity of fluoropolymer nanoparticles presents certain substantial challenges, such as maintaining a uniform dispersion. Consequently, the fluoropolymer nanoparticles are dispersed in a solvent system that inhibits agglomeration. The solvent for electrolytic plating compositions is typically water. When dispersed in water, the hydrophobic fluoropolymer nanoparticles tend to agglomerate into clumps having mean particle sizes greater than the mean particle size of the nanoparticles individually. This is disadvantageous because the larger agglomerated nanoparticles negatively impact the appearance and functionality of the silver-based composite coating. In other words, a silver-based composite coating that is glossy without the nanoparticles may be matte if it contains agglomerated clumps of nanoparticles. Also, large agglomerates of nanoparticles result in a higher co-deposition rate in the composite coating. Higher deposition rates of nonconductive fluoropolymer particle can negatively impact contact resistance. Accordingly, the solvent system for dispersing the fluoropolymer nanoparticles comprises a surfactant to inhibit agglomeration of the nanoparticles in aqueous solution.

Generally, the fluoropolymer nanoparticles are added to the electrolytic plating composition as a dispersion stabilized with a surfactant. In other words, the fluoropolymer nanoparticles are stabilized in a dispersion with a surfactant prior to combining other components (i.e., metal salts, complexing agent, water, etc.) of the electrolytic plating composition.

Dispersions of fluoropolymer nanoparticles are commercially available. One example of a dispersion of fluoropolymer nanoparticles is TEFLON PTFE 30 (available from DuPont), which is a dispersion of PTFE nanoparticles on the order of the wavelength of visible light or smaller. PTFE 30 comprises a dispersion of PTFE nanoparticles in water at a concentration of about 60 wt. % (60 grams of particles per 100 grams of solution) in which the particles have a particle size distribution between about 50 and about 500 nm, and a mean particle size of about 220 nm. Another example of a dispersion of fluoropolymer nanoparticles includes DRYFILM WD-4560 (available from DuPont), which is a dispersion of PTFE nanoparticles in water at a concentration of about 48 wt. % in which the particles have a mean particle size of about 80 nm. These particles are typically dispersed in a water/alcohol solvent system with a surfactant. Generally, the alcohol is a water soluble alcohol, having from 1 to about 4 carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, and tert-butanol. Typically, the ratio of water to alcohol (mole:mole) is between about 10 moles of water and about 20 moles of water per one mole of alcohol, more typically between about 14 moles of water and about 18 moles of water per one mole of alcohol.

Alternatively, a dispersion of fluoropolymer nanoparticles to be added to the electroplating composition of the present invention may be prepared by mixing dry fluoropolymer particles, solvent, and surfactant. An exemplary source of dry PTFE particles is TEFLON TE-5069AN, which comprises dry PTFE particles having a mean particle size of about 80 nm. Other sources of PTFE particles include the ALGOFLON series products by Solvay, and the DYNEON series products available from 3M of St. Paul, Minn. (U.S.).

As noted, the solvent system for dispersing the nanoparticles comprises one or more surfactants to inhibit agglomeration of the nanoparticles in aqueous solution and maintain the stability of the dispersion. The surfactant may be cationic, anionic, nonionic, or zwitterionic. A particular surfactant may be used alone or in combination with other surfactants.

Generally, surfactants comprise a hydrophilic head group and a hydrophobic tail. Hydrophilic head groups associated with anionic surfactants include carboxylate, sulfonate, sulfate, phosphate, and phosphonate. Hydrophilic head groups associated with cationic surfactants include quaternary amine, sulfonium, and phosphonium. Quaternary amines include quaternary ammonium, pyridinium, bipyridinium, and imidazolium. Hydrophilic head groups associated with nonionic surfactants include alcohol and amide. Hydrophilic head groups associated with zwitterionic surfactants include betaine. The hydrophobic tail typically comprises a hydrocarbon chain. The hydrocarbon chain typically comprises between about six and about 24 carbon atoms, more typically between about eight to about 16 carbon atoms.

In various embodiments, the dispersion of fluoropolymer nanoparticles comprises a nonionic surfactant. One class of nonionic surfactants includes those comprising polyether groups, based on, for example, repeating units of ethylene oxide (EO) and/or propylene oxide (PO). Surfactants having a polyether chain may comprise between about 1 and about 36 EO repeat units, between about 1 and about 36 PO repeat units, or a combination of between about 1 and about 36 EO repeat units and PO repeat units. More typically, the polyether chain comprises between about 2 and about 24 EO repeat units, between about 2 and about 24 PO repeat units, or a combination of between about 2 and about 24 EO repeat units and PO repeat units. Even more typically, the polyether chain comprises between about 6 and about 15 EO repeat units, between about 6 and about 15 PO repeat units, or a combination of between about 6 and about 15 EO repeat units and PO repeat units. These surfactants may comprise blocks of EO repeat units and PO repeat units, for example, a block of EO repeat units encompassed by two blocks of PO repeat units or a block of PO repeat units encompassed by two blocks of EO repeat units. Another class of polyether surfactants comprises alternating PO and EO repeat units. Within these classes of surfactants are the polyethylene glycols, polypropylene glycols, and the polypropylene glycol/polyethylene glycols.

Another class of nonionic surfactants comprises EO, PO, or EO/PO repeating units built upon an alcohol or phenol base group, such as glycerol ethers, butanol ethers, pentanol ethers, hexanol ethers, heptanol ethers, octanol ethers, nonanol ethers, decanol ethers, dodecanol ethers, tetradecanol ethers, phenol ethers, alkyl substituted phenol ethers, α-naphthol ethers, and β-naphthol ethers. With regard to the alkyl substituted phenol ethers, the phenol group is substituted with a hydrocarbon chain having between about 1 and about 10 carbon atoms, such as about 8 (octylphenol) or about 9 carbon atoms (nonylphenol). The polyether chain may comprise between about 1 and about 24 EO repeat units, between about 1 and about 24 PO repeat units, or a combination of between about 1 and about 24 EO and PO repeat units. More typically, the polyether chain comprises between about 8 and about 16 EO repeat units, between about 8 and about 16 PO repeat units, or a combination of between about 8 and about 16 EO and PO repeat units. Even more typically, the polyether chain comprises about 9, about 10, about 11, or about 12 EO repeat units; about 9, about 10, about 11, or about 12 PO repeat units; or a combination of about 9, about 10, about 11, or about 12 EO repeat units and PO repeat units.

An exemplary β-naphthol derivative nonionic surfactant is LUGALVAN BN012 which is a β-naphtholethoxylate having 12 ethylene oxide monomer units bonded to the naphthol hydroxyl group. A similar surfactant is POLYMAX NPA-15, which is a polyethoxylated nonylphenol. Another surfactant is TRITON-X100 nonionic surfactant, which is an octylphenol ethoxylate, typically having around 9 or 10 EO repeat units. Additional commercially available nonionic surfactants include the PLURONIC P, L, and F series of surfactants. PLURONIC surfactants include the P series of EO/PO block copolymers, including P65, P84, P85, P103, P104, P105, and P123; the F series of EO/PO block copolymers, including F108, F127, F38, F68, F77, F87, F88, F98; and the L series of EO/PO block copolymers, including L10, L101, L121, L31, L35, L44, L61, L62, L64, L81, and L92. These surfactants are available from BASF.

Additional commercially available nonionic surfactants include water soluble, ethoxylated nonionic fluorosurfactants available from DuPont and sold under the trade name ZONYL, including ZONYL FSN (telomar B monoether with polyethylene glycol nonionic surfactant), ZONYL FSN-100, ZONYL FS-300, ZONYL FS-500, ZONYL FS-510, ZONYL FS-610, ZONYL FSP, and ZONYL UR. Other nonionic surfactants include the amine condensates, such as cocoamide DEA and cocoamide MEA. Other classes of nonionic surfactants include acid ethoxylated fatty acids (polyethoxy esters) comprising a fatty acid esterified with a polyether group typically comprising between about 1 and about 36 EO repeat units. Glycerol esters comprise one, two, or three fatty acid groups on a glycerol base.

In various embodiments, the dispersion of fluoropolymer nanoparticles comprises an anionic surfactant. Exemplary anionic surfactants include alkyl phosphonates, alkyl ether phosphates, alkyl sulfates, alkyl ether sulfates, alkyl sulfonates, alkyl ether sulfonates, carboxylic acid ethers, carboxylic acid esters, alkyl aryl sulfonates, and sulfosuccinates. Anionic surfactants include any sulfate ester, such as those sold under the trade name ULTRAFAX (available from MFG Chemical Inc.), including, sodium lauryl sulfate, sodium laureth sulfate (2 EO), sodium laureth, sodium laureth sulfate (3 EO), ammonium lauryl sulfate, ammonium laureth sulfate, TEA-lauryl sulfate, TEA-laureth sulfate, MEA-lauryl sulfate, MEA-laureth sulfate, potassium lauryl sulfate, potassium laureth sulfate, sodium decyl sulfate, sodium octyl/decyl sulfate, sodium 2-ethylhexyl sulfate, sodium octyl sulfate, sodium nonoxynol-4 sulfate, sodium nonoxynol-6 sulfate, sodium cumene sulfate, and ammonium nonoxynol-6 sulfate. Anionic surfactants also include sulfonate esters such as sodium α-olefin sulfonate, ammonium xylene sulfonate, sodium xylene sulfonate, sodium toluene sulfonate, dodecyl benzene sulfonate, and lignosulfonates. Additional anionic surfactants include sulfosuccinate surfactants such as disodium lauryl sulfosuccinate, disodium laureth sulfosuccinate; and others including sodium cocoyl 2-hydroxyethanesulfonate, lauryl phosphate, any of the ULTRAPHOS series of phosphate esters (available from MFG Chemical Inc.), CYASTAT 609 (N,N-bis(2-hydroxyethyl)-N-(3′-dodecyloxy-2′-hydroxypropyl) methyl ammonium methosulfate) and CYASTAT LS (3-lauramidopropyl) trimethylammonium methylsulfate), which available from Cytec Industries.

In various embodiments, the dispersion of fluoropolymer particles comprises a cationic surfactant. Exemplary cationic surfactants include quaternary ammonium salts such as dodecyl trimethyl ammonium chloride, cetyl trimethyl ammonium salts of bromide and chloride, hexadecyl trimethyl ammonium salts of bromide and chloride, alkyl dimethyl benzyl ammonium salts of chloride and bromide, and the like. In this regard, surfactants such as S-106A (fluoroalkyl ammonium chloride cationic surfactant 28-30% and S-208M (blend anionic and cationic fluoroalkyl surfactants 33%, with a net positive charge), available from Chemguard Specialty Chemicals & Equipment, and AMMONYX 4002 (octadecyl dimethyl benzyl ammonium chloride cationic surfactant, available from Stepan Company, Northfield, Ill.) are suitable.

In accordance with the present invention, the electrolytic plating composition comprises an auxiliary surfactant. Upon mixing of the fluoropolymer nanoparticle dispersion in the plating composition, the pre-mix surfactant present in the dispersion is diluted by the plating composition solution such that the fluoropolymer nanoparticles may destabilize and agglomerate. Incorporating an effective amount of an auxiliary surfactant into the plating composition has been found to maintain the stability the dispersion fluoropolymer nanoparticles by preventing this agglomeration of the particles. The auxiliary surfactant may also promote wetting of the substrate surface and modify the surface tension of the electrolytic plating solution. Typically, the auxiliary surfactant comprises a nonionic or cationic surfactant. The auxiliary surfactant may be any nonionic or cationic surfactant previously mentioned as suitable for incorporation into the fluoropolymer nanoparticle dispersion.

As previously explained, the fluoropolymer nanoparticles are co-deposited with the silver or silver alloy metal on the substrate surface. The nanoparticles are not reduced, but are trapped at the cathode surface interface by the silver or silver alloy ions, which are reduced and deposited around the fluoropolymer nanoparticle-surfactant complex. The auxiliary surfactant may be chosen to impart a predominantly positive charge to the nanoparticles, which facilitates migration of the nanoparticles toward the cathode and lightly adheres them to the surface until encapsulated and trapped on the surface by deposited metal.

In various embodiments, the auxiliary surfactant comprises a cationic surfactant that imparts an overall positive surfactant charge to the fluoropolymer nanoparticles. A positively charged fluoropolymer nanoparticle-surfactant complex will tend to drive the particles toward the cathode substrate during electrolytic deposition to a greater degree than other neutrally charged complex.

The overall charge of the fluoropolymer nanoparticle-surfactant complex may be quantified as follows. The charge of a particular surfactant molecule is typically −1 (anionic), 0 (nonionic or zwitterionic), or +1 (cationic). A population of surfactant molecules therefore has an average charge per surfactant molecule that ranges between −1 (entire population comprises anionic surfactant molecules) and +1 (entire population comprise cationic surfactant molecules). A population of surfactant molecules having an overall 0 charge may comprise, for example, 50% anionic surfactant molecules and 50% cationic surfactant molecules, 100% zwitterionic surfactant molecules, or 100% nonionic surfactant molecules.

Upon mixing of the fluoropolymer nanoparticle dispersion and the auxiliary surfactant in the electrolytic plating composition, the plating composition includes a combination of cationic, anionic and/or nonionic surfactant molecules with other anionic, zwitterionic, cationic, and/or nonionic surfactant molecules. Preferably, the average charge per surfactant molecule of the population of surfactant molecules in the complex is greater than 0. In certain embodiments, the electrolytic plating composition comprises a cationic auxiliary surfactant used in combination with one or more pre-mix cationic surfactants and/or one or more pre-mix nonionic surfactants. Accordingly, in various embodiments, the surfactant mixture comprises cationic surfactant molecules and nonionic surfactant molecules preferably having an average charge per surfactant molecule between about 0.01 (99% nonionic surfactant molecules and 1% cationic surfactant molecules) and 1 (100% cationic surfactant molecules) or between about 0.1 (90% nonionic surfactant molecules and 10% cationic surfactant molecules) and 1. The average charge per surfactant molecule of the population of surfactant molecules making up the surfactant mixture in the plating composition (pre-mix surfactant+auxiliary surfactant) may be at least about 0.2 (80% nonionic surfactant molecules and 20% cationic surfactant molecules), such as at least about 0.3 (70% nonionic surfactant molecules and 30% cationic surfactant molecules), at least about 0.4 (60% nonionic surfactant molecules and 40% cationic surfactant molecules), at least about 0.5 (50% nonionic surfactant molecules and 50% cationic surfactant molecules), at least about 0.6 (40% nonionic surfactant molecules and 60% cationic surfactant molecules), at least about 0.7 (30% nonionic surfactant molecules and 70% cationic surfactant molecules), at least about 0.8 (20% nonionic surfactant molecules and 80% cationic surfactant molecules), or even at least about 0.9 (10% nonionic surfactant molecules and 90% cationic surfactant molecules). In each of these embodiments, the average charge per surfactant molecule is no greater than 1.

The concentration of the auxiliary surfactant may be determined by the total particle-matrix interface area. For a given weight concentration of the particle, the smaller the mean particle size, the higher the total area of the particle surface. The total surface area is calculated by the specific particle surface (m²/g) multiplied by the particle weight in the solution (g). The calculation yields a total surface area in m². A given concentration of fluoropolymer nanoparticles, having a high specific particle surface area, includes a much greater total number of particles compared to micrometer-sized particles of the same weight concentration. Therefore, high concentrations of surfactants are used to decrease the tendency of the nanoparticles to flocculate or coagulate. The auxiliary surfactant concentration is therefore a function of the mass and specific surface area of the particles. Accordingly, in various embodiments, the composition comprises about one gram of auxiliary surfactant for every about 10 m² to about 450 m², about 10 m² to about 250 m², about 20 m² to about 150 m², about 20 m² to about 150 m², about 20 m² to about 80 m², or about 30 m² to about 75 m² of surface area of fluoropolymer particles.

For example, DRYFILM WD-4560 is a PTFE nanoparticle dispersion available from DuPont that contains approximately 48 wt. % of PTFE particles having an average particle size of about 80 nm and a specific surface area of about 23.0 m²/g. The mass of auxiliary surfactant to maintain the dispersion of one gram of these PTFE particles is from about 0.05 g to about 2 g, from about 0.1 g to about 2 g, from about 0.2 g to about 1.5 g, or from about 0.3 g to about 1 g (e.g. about 0.3 g).

The auxiliary surfactant concentration may also be specified on a molar basis per unit of fluoropolymer particle surface area. The auxiliary surfactant molar concentration may range from at least about 0.0001 up to about 1 mmoles of surfactant per m² of fluoropolymer particle surface area. In various embodiments, the auxiliary surfactant molar concentration is from about 0.0005 to about 0.5, from about 0.001 to about 0.5, or from about 0.005 to about 0.1 mmoles of surfactant per m² of fluoropolymer particle surface area. In certain embodiments, the auxiliary surfactant molar concentration is from about 0.0125 to about 1, from about 0.015 to about 0.5, from about 0.05 to about 0.25, from about 0.075 to about 0.15, or from about 0.02 to about 0.1 mmoles of surfactant per m² of fluoropolymer particle surface area. In other embodiments, the auxiliary surfactant molar concentration is from about 0.0001 to about 0.002 or from about 0.0005 to about 0.002 mmoles of surfactant per m² of fluoropolymer particle surface area.

In certain embodiments, the auxiliary surfactant comprises a cationic fluorosurfactant. Incorporating an effective amount of cationic fluorosurfactant into the plating composition has been found to not only stabilize the fluoropolymer dispersion, but provide stability over an extended period of time (e.g., greater than one month). Also, surprising, a cationic fluorosurfactant provides a greater degree of co-deposition of fluoropolymer particles with silver when compared to other surfactants and improves the appearance of the deposit by enhancing the brightness and color. In some embodiments, the auxiliary surfactant comprises a cationic fluorosurfactant, wherein the electrolytic plating composition is free of any nonionic fluorosurfactants or is essentially free of any nonionic fluorosurfactants (i.e., no more than about 0.00005 or about 0.00001 mmoles of nonionic fluorosurfactant per m² of fluoropolymer particle surface area). In certain embodiments, the auxiliary surfactant consists or consists essentially of a cationic fluorosurfactant or a mixture of anionic and cationic fluorosurfactants. In still further embodiments, the only fluorosurfactant contained in the electrolytic plating composition is a cationic fluorosurfactant. In some embodiments, the electrolytic plating composition free of other cationic surfactants or essentially free of other cationic surfactants (i.e., no more than about 0.00005 or about 0.00001 mmoles of nonionic fluorosurfactant per m² of fluoropolymer particle surface area) besides a cationic fluorosurfactant.

Fluorosurfactants include, for example, fluoroalkyl ammonium salts of halides (e.g., bromide and chloride). Specific fluorosurfactants are S-106A (fluoroalkyl ammonium chloride cationic surfactant 28-30% with 10% hexylene glycol) and S-208M (blend anionic and cationic fluoroalkyl surfactants 33%, with a net positive charge), which are available from Chemguard Specialty Chemicals & Equipment. In some embodiments, the auxiliary surfactant consists or consists essentially of a fluoroalkyl ammonium chloride cationic surfactant.

It has been found that incorporating a relatively high ratio of cationic fluorosurfactant to fluoropolymer nanoparticle surface area provides greater stability to the plating composition. Accordingly, in some embodiments, the composition comprises about one gram of cationic fluorosurfactant for every about 10 m² to about about 70 m² about of surface area of fluoropolymer particle. In certain embodiments, the concentration of the cationic fluorosurfactant in the electrolytic plating composition is from about 0.2 g/L to about 35 g/L, from about 0.5 g/L to about 20 g/L, from about 0.5 g/L to about 10 g/L, from about 1 g/L to about 6 g/L, from about 1 g/L to about 5 g/L, from about 1 g/L to about 4 g/L, from about 1 g/L to about 3 g/L, or from about 1.5 g/L to about 6 g/L.

The electrolytic plating composition of the present invention may comprise other additives including, for example, wetting agents, conductive salts, brighteners, complexing agents, pH adjusters, and buffering agents.

In various embodiments, the electrolytic plating composition of the present invention comprises a wetting agent. A wetting agent is added to the electrolytic plating composition to promote wetting of the substrate surface and modify the surface tension of the electrolytic plating solution. With regard to the plating process, a plating solution with a low surface tension advantageously: (1) enhances wetting of the substrate surface; (2) enhances the ability of the solution to reduce or eliminate gas bubbles; (3) prevents pits/voids on the plated surface; (4) increases the solubility of organic materials such as grain refiners, brighteners, and other bath additives; and (5) lowers the deposition potentials of various metals which allows for uniform deposits and alloys. A plating solution with a low surface tension is particularly advantageous with regard to the fluoropolymer nanoparticles because this enhances the dispersability of the fluoropolymer nanoparticles in the plating composition.

Suitable wetting agents for use in the present invention include, for example, sulfonic acid derivatives such as sulfopropylated polyalkoxylated beta-naphthol, condensation products of naphthalenesulfonic acid, and salts thereof (e.g., sodium polynaphthaleneformaldehyde sulfonate). Specific examples of wetting agents include the TAMOL N series (e.g., TAMOL NN 9401 and TAMOL NN 8906) and the VULTAMOL NN series that are available from BASF.

Typically, the electrolytic plating composition contains at least about 5 g/L, at least about 10 g/L, at least about 15 g/L, or at least about 20 g/L of a wetting agent. In various embodiments, the electrolytic plating composition contains from about 5 g/L to about 50 g/L, from about 5 to about 40 g/L, from about 10 g/L to about 30 g/L, or from about 10 g/L to about 20 g/L of a wetting agent.

The electrolytic plating composition of the present invention may further comprise a conductive salt comprising a sulfonic acid and/or a derivative of a sulfonic acid at a concentration between 50 g/L and 500 g/L, between 100 g/L and 300 g/L, or between 130 g/L and 200 g/L. Preferably, the conductive salt comprises potassium methane sulfonate. In addition to potassium methane sulfonate, other methane sulfonates such as sodium methane sulfonate are suited, but also sulfates and other compounds are suitable as a conductive salt for use in the electrolytic plating composition.

An effective amount of alkali bromide can be added to the electrolytic plating composition, for improving the deposition result (e.g., from bright to matte depending on the concentration). The addition of alkali bromides, especially potassium bromide, generally provides a more uniform appearance of the composite coating on the substrate surface. When a uniform dull layer is desired, potassium bromide is particularly suitable. Moreover, concerning the color, more uniform deposition results are obtained by the addition of alkali bromides such as potassium bromide. Accordingly, in various embodiments, the electrolytic plating composition comprises from about 30 mg/L to about 500 mg/L or from about 100 mg/L to about 200 mg/L of alkali bromide. Other additives such as bismuth citrate and selenium salts can also be added to improve the surface brightness.

In some embodiments, the electrolytic plating composition includes a thiosulfate such as sodium thiosulfate. The thiosulfate is typically added to the electrolytic plating composition at a concentration from about 50 mg/L to about 500 mg/L or from about 100 mg/L to about 200 mg/L. Here, the thiosulfate serves as a matting agent. The silver layers deposited from such an electrolytic plating composition are uniformly dull, almost free of internal stresses, and exhibit excellent soldering properties.

In various embodiments, the electrolytic plating composition further comprises an antifoam additive. For example, silicone-based emulsion antifoams may be used to control the amount of foam generated during use of the plating composition. One suitable silicone-based antifoam additive is DC 1430, which is available from Dow Corning. When used, the antifoam additive is typically added to the electrolytic plating composition in an amount from about 10 to about 500 ppm or from about 30 to about 300 ppm.

In certain embodiments, the electrolytic plating composition of the present invention comprises the following:

Silver (as silver methanesulfonate, Ag-MSA) 10-50 g/L Hydantoin compound (e.g., 5,5-dimethylhydantoin) 80-150 g/L Conductive salt (e.g., potassium methanesulfonate) 50-500 g/L Brightener (e.g., potassium bromide) 30-500 mg/L PTFE dispersion (e.g., 48 wt. % PTFE with non- 5-200 g/L ionic C₉₋₁₁, 6EO alcohol ethoxylate surfactant) Cationic fluorosurfactant (e.g., S-106A) 0.2-35 g/L Wetting agent (e.g., TAMOL NN 9401 or 5-50 g/L TAMOL NN 8906, a sodium salt of a naphthalenesulfonic acid condensation product) Antifoam agent (e.g., DC 1430; silicone based 30-300 ppm antifoam additive) The concentrations of each component may be varied independently within the specified ranges set forth above.

The pH of the electrolytic plating composition according to the invention is from about 8 to about 14, from about 9 to about pH 12.5, from about 9.5 to about 11.5, or from about 9 to about 11. Base, such as potassium hydroxide, may be used to maintain the pH of the composition.

The electrolytic plating composition can be prepared by the following process. First, the complexing agent (e.g., a hydantoin compound), any wetting agent and conductive salt (e.g., potassium methane sulfonate) are mixed with water to form a solution. The silver ion source (e.g., Ag-MSA) is added to the solution followed by the cationic fluorosurfactant and any alkali bromide. Then, the pre-mix dispersion comprising fluoropolymer nanoparticles particles and surfactant is mixed in the solution along with any antifoam additive needed. To avoid precipitation, the pH of the solution is maintained throughout the process from about 9 to about 11 (e.g., about 10). Base, such as potassium hydroxide, may be added as needed to maintain the pH of the solution.

The plating composition of the present invention is used in a process for electrolytically depositing a silver-based composite coating on a substrate. Electrolytic deposition occurs by contacting the surface of the substrate with the electrolytic plating composition. The cathode substrate and anode are electrically connected by wiring and, respectively, to a rectifier. That is, an external source of electrons is applied to the electrolytic plating composition to thereby electrolytically deposit the composite coating onto the substrate surface. The cathode substrate has a net negative charge so that deposition metal ions in the solution are reduced at the cathode substrate depositing the silver-based composite coating on the cathode surface. An oxidation reaction takes place at the anode. The nanoparticles are trapped at the interface by the metal ions, which are reduced and deposited around the nanoparticle. The cathode and anode may be horizontally or vertically disposed in the tank.

During operation of the electrolytic plating system, deposition metal ions are reduced onto the surface of a cathode substrate when the rectifier is energized. A pulse current, direct current, reverse periodic current, or other suitable current may be employed. Typically, the substrate to be coated is contacted with the electrolytic plating composition at a set current density from about 0.1 to about 10 A/dm², from about 1 to about 5 A/dm², or from about 2.5 to about 3.5 A/dm². The silver-based composite coating can be deposited at a plating rate from about 0.05 μm/min to about 5 μm/min, from about 0.5 μm/min to about 5 μm/min, from about 1 μm/min to about 5 μm/min, or from about 1 μm/min to about 2.5 μm/min.

The temperature of the electrolytic solution may be maintained using a heater/cooler whereby electrolytic solution is removed from the holding tank and flows through the heater/cooler and then is recycled to the holding tank. Typical operating temperatures of the electrolytic composition range from about 40° C. to about 60° C., from about 40° C. to about 55° C., or from about 50° C. to about 55° C.

The silver-based composite coating of the present invention may be applied to a variety of substrates. Exemplary substrates for coating with the silver-based composite coatings of the present invention include electrical connectors and other electronics parts, automotive parts, metallized plastics, and non-stick parts for use in injection molding tools. In some embodiments, the substrate for coating includes electrical connectors and, more particularly, copper or copper alloy connectors with or without a nickel barrier layer.

A silver-based composite coating comprising fluoropolymer nanoparticles deposited using the electrolytic plating composition of the present invention may have thickness that is no greater than about 20 μm, no greater than about 10 μm, no greater than about 5 μm, or no greater than about 3 μm. For example, in some embodiments, the thickness of silver-based composite coating is from about 0.5 μm to about 10 μm, from about 1 μm to about 10 μm, or from about μm to about 5 μm (e.g., about 1 μm to about 3 μm).

A silver-based composite coating comprising fluoropolymer nanoparticles deposited using the electrolytic plating composition of the present invention typically has a coefficient of friction that is significantly reduced when compared to a silver coating prepared from a similar plating composition that does not contain fluoropolymer nanoparticles. Accordingly, in various embodiments, the silver-based composite coating has a coefficient of friction from about 0.05 to about 0.5, from about 0.05 to about 0.3, or from about 0.1 to about 0.2.

The silver-based composite coating comprising fluoropolymer nanoparticles deposited using the electrolytic plating composition of the present invention typically contains at least 1.2, 1.5, 1.8, 2.0, 2.5, 3.0, 3.5 or 4.0 wt. % fluorine (atomic), which can be determined by energy-dispersive x-ray spectroscopy (EDS). In particular embodiments, the silver-based composite coating contains from about 1 wt. % to about 7 wt. %, from about 1 wt. % to about 5 wt. %, from about 1 wt. % to about 3 wt. %, from about 2 wt. % to about 7 wt. %, from about 2.5 wt. % to about 5 wt. %, or from about 3 wt. % to about 4 wt. % fluorine (atomic).

The electrolytic plating composition of the present invention provides silver-based composite coatings that have a contact resistance that is comparable to silver-based coatings that do not contain fluoropolymer nanoparticles. Accordingly, the silver-based composite coatings typically have a contact resistance that is less than about 10 milliohms, less than about 5 milliohms, or less than about 2 milliohms. Further, unlike pure silver deposits, the contact resistance of the silver-based composite coating comprising fluoropolymer nanoparticles deposited using the electrolytic plating composition of the present invention remain constant or nearly constant even after thermal aging and with no nickel barrier layer present. For example, the silver-based composite coatings have a contact resistance that is less than about 10 milliohms, less than about 5 milliohms, or less than about 2 milliohms after 100 hours at 150° C. under a static load between 50 and 250 grams.

The electrolytic plating composition of the present invention provides silver-based composite coatings that maintain a uniform appearance even after thermal aging. Pure silver deposits on copper or copper alloy substrates discolor due to copper diffusion to the surface of the silver deposit. However, the silver-based composite coatings of the present invention restrict or eliminate copper diffusion such that the appearance of the deposit remains uniform over time and exposure to heat.

Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.

EXAMPLES

The following non-limiting examples are provided to further illustrate the present invention.

Example 1

The following plating compositions were prepared for the deposition of silver and silver-based composite coatings. The pH of each composition was approximately 9.5. Potassium hydroxide was added as need to maintain this pH.

Bath 1-1 Silver (as Ag-MSA) 30 g/L 5,5-dimethylhydantoin 130 g/L TAMOL NN 9401 20 g/L (sodium salt of a naphthalenesulfonic acid condensation product) potassium methanesulfonate 100 g/L water balance to 1 L

Bath 1-2 Silver (as Ag-MSA) 30 g/L 5,5-dimethylhydantoin 130 g/L TAMOL NN 9401 20 g/L potassium methanesulfonate 100 g/L DRYFILM WD-4560 (48% PTFE 10 g/L (PTFE mass dispersion with <5% nonionic concentration) surfactant) water balance to 1 L

Bath 1-3 Silver (as Ag-MSA) 30 g/L 5,5-dimethylhydantoin 125 g/L TAMOL NN 9401 20 g/L (sodium salt of a naphthalenesulfonic acid condensation product) potassium methanesulfonate 100 g/L potassium bromide approx. 100-160 ppm S-106A (cationic fluorosurfactant) 3 mL/L DC 1430 (antifoam additive) approx. 30-300 ppm water balance

Bath 1-4 Silver (as Ag-MSA) 30 g/L 5,5-dimethylhydantoin 125 g/L TAMOL NN 9401 20 g/L (sodium salt of a naphthalenesulfonic acid condensation product) potassium methanesulfonate 100 g/L potassium bromide approx. 100-160 ppm S-106A (cationic fluorosurfactant) 3 mL/L DC 1430 (antifoam additive) approx. 30-300 ppm DRYFILM WD-4560 (48% PTFE 10 g/L (PTFE mass dispersion with <5% nonionic concentration) surfactant) water balance

Copper alloy substrates were contacted at a set current density of 2.6 A/dm² (1.2μ/min) for approximately one minute with each bath. The coefficient of friction (COF) and contact resistance (CR) of each coating deposited from these plating baths were measured using a CETR universal micro-tribometer. A 50 g load was applied for 500 cycles to measure the coefficient of friction. A pure bright silver plated ball was used during the contact resistance measurements. The weight percent of atomic fluorine in the coatings were determined by EDS. The results of these measurements are presented in the following table.

Ag- PTFE coating Plating Conc. Thickness CR Composition (g/L) (microns) wt. % F COF (milliohm) Bath 1-1 — 1.36 — 0.57 1.6 Bath 1-2 10 1.36 2 0.11 2.2 Bath 1-3 — 1.2 — 0.91 5.3 Bath 1-4 10 1.2 4.04 0.10 5.44

The coating surfaces prepared from Bath 1-3 and Bath 1-4 with PTFE nanoparticles were also evaluated for wear resistance after the coefficient of friction test, which applied a 50 g load over the course of 500 sliding cycles. The profiles of each surface were measured using a profilometer. Profile plots of each surface are provided in FIGS. 1A and 1B, respectively. The width of the wear track on the silver coating prepared from Bath 1-3 was 290 microns at a depth of approximately 1.8 microns. The width of the wear track for the silver composite coating prepared from Bath 1-4 with PTFE nano-particles was 186 microns at a depth of about 0.4 microns.

The results from these tests show that the compositions of the present invention significantly reduce the coefficient of friction of a silver coating and improve wear resistance while maintaining a low contact resistance.

Example 2

A silver composite coating was deposited on a copper alloy substrate using Bath 1-4 prepared in accordance with Example 1. The coated substrate was then heated at 125° C. for seven days. Coefficient of friction and contact resistance measurements as described in Example 1 were repeated after the thermal aging. The results show that these properties did not change after thermal aging.

Example 3

A solution of silver methanesulfonate was mixed with a PTFE dispersion at a concentration of 10 g/L of PTFE. Samples of the solution were mixed with four different surfactants. The stability of the solutions was examined after five weeks of shelf aging. The results are provided in the following table.

Auxiliary Surfactant Stability Composition Conc. Observations Sample Auxiliary Surfactant (mL/L) After 5 weeks Control — — Multi-phase (Ag-MSA + solution; PTFE PTFE coagulated and dispersion) separated 3-1 cetyltrimethylammonium 3 Homogeneous chloride solution 3-2 S106A 3 Homogeneous (fluoroalkyl ammonium solution chloride cationic surfactant 28-30%) 3-3 S208M 3 Homogeneous (fluoroalkyl ammonium solution chloride cationic/anionic surfactant blend 33%) 3-4 NEODOL 91-6 (C₉₋₁₁, 3 Homogeneous 6EO alcohol ethoxylate) solution

Example 4

Copper alloy substrates were coated with a silver composite coating using compositions prepared in accordance with Example 3. The substrates were coated after periods of shelf aging of the plating composition. The weight percent of atomic fluorine (w % F) was determined by EDS as a measure of PTFE nanoparticle co-deposition rate. A plot of the aging time vs. the w % F is presented in FIG. 2. Fluorosurfactant S106A provided the highest rate of PTFE nanoparticle co-deposition during the entire aging test period.

Example 5

A series of copper alloy substrates were coated with a silver composite coating using Bath 1-4 prepared in accordance with Example 1, except that the concentration of PTFE nanoparticles was varied from 0 g/L to 40 g/L. The appearance of the coatings was observed after coating (freshly made) and after a thermal aging test: (1) after 168 hours of heating at 125° C. or (2) after 100 hours of heating at 150° C. FIG. 3. shows images of the surface of each substrate after each period. The pure silver coatings and the composite coatings deposited from the plating composition containing 1 g/L of PTFE nanoparticles showed discoloration due in part to copper diffusion to the surface of the silver coating. The substrates having silver composite coatings deposited from plating compositions containing 5 g/L or more of PTFE nanoparticles had a uniform appearance even after the thermal aging tests.

FIG. 4 shows scanning electron microscope (SEM) images of each surface after coating (freshly made). The SEM images show that the PTFE particles are uniformly distributed throughout the coating surface (the dark spots are the PTFE particles in the images featured in FIGS. 4-9). FIG. 5 provides a SEM image of the composite silver coating prepared from the bath containing 40 g/I PTFE. The image shows that PTFE particles are uniformly distributed three-dimensionally, throughout the coating. FIG. 6 shows SEM images of the coated surface grain structures after 168 hours of heating at 125° C. FIG. 7 shows SEM images of the coated surface grain structures after 100 hours of heating at 150° C. FIG. 8 shows focused ion beam (FIB) images of a cross-section of selected samples after 100 hours of heating at 150° C. Hollow spaces were identified along the copper substrate surface and silver interface in the pure silver deposit, possibly caused by copper diffusion through the silver layer. The composite silver coating prepared from the bath containing 5 g/I PTFE exhibited a tight fitting substrate surface and silver-PTFE interface. FIG. 9 shows focused ion FIB images of a cross-section of selected samples after 100 hours of heating at 150° C. Diffused copper particles are present on the surface of the pure silver deposit (0 g/L PTFE), which are shown as light colored spots near the surface of the deposit. The composite silver coating prepared from the bath containing 10 g/I PTFE appears to effectively prevent copper diffusion to the surface of the deposit, which is exhibited a tight fitting substrate surface and silver-PTFE interface.

Contact resistance of each coating was measured using a CETR universal micro-tribometer and a QUadTech LR2000 Milliohmmeter. The tribometer controlled the applied load, which was increased from 0 to 250 g. A pure bright silver plated ball was used during the contact resistance measurements. The contact resistance measurements for each freshly made substrate (no thermal aging) are presented in FIG. 10A and Table 1. A magnified selection of the FIG. 10A plot between a contact resistance of 1 and 2 milliohms is presented in FIG. 10B. The contact resistance measurements for each substrate after 100 hours of heating at 150° C. are presented in FIG. 10C and Table 2. The contact resistance measurements for each substrate after 168 hours of heating at 125° C. are presented in FIG. 10D and Table 3. A magnified selection of the FIG. 10D plot between a contact resistance of 1 and 4 milliohms is presented in FIG. 10E.

TABLE 1 Contact resistance measurements for freshly coated substrates (no thermal aging) Contact Resistance (milliohm) Load Pure Ag 1 g 5 g 10 g 20 g 40 g (g) (0 PTFE) PTFE PTFE PTFE PTFE PTFE 50 1.532 1.543 1.569 1.604 1.591 1.805 100 1.380 1.393 1.396 1.433 1.415 1.538 150 1.310 1.373 1.320 1.346 1.348 1.396 200 1.263 1.277 1.300 1.306 1.332 1.365 250 1.303 1.283 1.300 1.263 1.268 1.310

TABLE 2 Contact resistance measurements for coated substrates after 100 hours at 150° C. Contact Resistance (milliohm) Load Pure Ag 1 g 5 g 10 g 20 g 40 g (g) (0 PTFE) PTFE PTFE PTFE PTFE PTFE 50 38.842 30.949 3.970 4.499 4.010 4.597 100 34.237 17.870 2.936 3.863 3.069 3.722 150 31.138 14.329 2.684 3.505 2.831 3.403 200 36.088 14.127 2.314 3.161 2.818 2.860 250 41.582 14.963 2.671 3.029 2.910 2.860

TABLE 3 Contact resistance measurements for coated substrates after 168 hours at 125° C. Contact Resistance (milliohm) Load Pure Ag 1 g 5 g 10 g 20 g 40 g (g) (0 PTFE) PTFE PTFE PTFE PTFE PTFE 50 3.167 1.777 2.076 1.828 2.109 2.592 100 2.164 1.545 1.604 1.511 1.815 1.931 150 1.764 1.463 1.482 1.343 1.558 1.743 200 1.579 1.374 1.342 1.296 1.466 1.644 250 1.573 1.354 1.379 1.285 1.408 1.569

Notably, after 100 hours of thermal aging at 150° C., the pure silver coatings and the composite coatings deposited from the plating composition containing only 1 g/L of PTFE nanoparticle each had a contact resistance above 10 milliohms. The contact resistances of substrates having silver composite coatings deposited from plating compositions containing 5 g/L or more of PTFE nanoparticles were consistently below 5 milliohms even after the thermal aging tests.

Example 6

The substrates from Example 5 having the pure silver coating and the composite coating deposited from the plating composition containing 10 g/L of PTFE nanoparticles, which had been subjected to thermal aging for 100 hours at 150° C. were analyzed using auger electron spectroscopy (AES). FIG. 11A presents the results of this analysis as a function of coating depth for the pure silver coating. FIG. 11B presents the results of this analysis as a function of coating depth for the composite coating deposited from the plating composition containing 10 g/L of PTFE nanoparticles. The results show that the pure silver coating contained a significant amount of copper at the surface of the silver deposit after thermal aging. On the other hand, the composite coating contained negligible amounts of copper at the surface. These results show that the composite coating was effective in restricting copper diffusion to the surface of the silver coating.

Example 7

Brass alloy substrates were plated in a flow cell for testing high speed plating applications. Three plating compositions were tested in this experiment. The cationic fluorosurfactant concentration was varied in each plating composition (i.e., 1.7 ml/L, 2.5 ml/L, or 3.3 ml/L). The plating composition contained the following ingredients:

Silver (as Ag-MSA) 35 g/L 5,5-dimethylhydantoin 125 g/L TAMOL NN 9401 20 g/L (sodium salt of a naphthalenesulfonic acid condensation product) potassium methanesulfonate 100 g/L potassium bromide 120 ppm S-106A (cationic fluorosurfactant, 1.7 ml/L, 2.5 ml/L, 3.3 ml/L 30 wt. %) DC 1430 (antifoam additive) 60 ppm DRYFILM WD-4560 (48% PTFE 5.6 g/L (PTFE mass dispersion with <5% nonionic concentration) surfactant) water balance In each test run, the flow cell vessel contained 8 liters of the plating composition. Spargers were located at the bottom of the flow cell vessel and were directed toward the face of the substrates. The plating compositions were agitated during use. The pH of each plating composition was maintained at about 10, and the temperature during plating was maintained at about 53° C. Each substrate was immersed in the plating composition for 1 minute while being shifted horizontally in the vessel to simulate high speed plating. The current density applied was approximately 2.5 A/dm². Observations were made of the foam level of each plating composition in use. Each composition had an acceptable foam level.

The thickness of the silver composite coating on the front and back of the substrates was measured by X-ray fluorescence. Also, the weight percent of atomic fluorine (w % F) was determined by EDS as a measure of PTFE nanoparticles co-deposition rate. The results are provided in Table 4 below.

TABLE 4 Thickness and atomic fluorine measurements for coated substrates 1.7 ml/L 2.5 ml/L 3.3 ml/L surfactant surfactant surfactant Front Back Front Back Front Back Thickness 1.4 1.67 1.46 1.55 1.48 1.63 (μm) w % F 2.76 2.83 2.72 2.66 2.37 2.43

The appearance of each substrate was compared. Images of the coated substrates are presented in FIG. 12. The coatings were uniform, however, the color of the coating deposited from the plating composition containing 1.7 ml/L of surfactant was yellowish in color. Increasing the surfactant concentration to 2.5 ml/L and 3.3 ml/L improved the color of the deposit. Also, this Example shows that the plating compositions are suitable for high speed plating applications.

When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above compositions and processes without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawing[s] shall be interpreted as illustrative and not in a limiting sense. 

1. An electrolytic plating composition for depositing a composite silver or silver alloy coating on a substrate comprising: (a) a silver ion source comprising silver methane sulfonate (Ag-MSA); (b) a complexing agent comprising a compound comprising a nitrogen-containing heterocyclic ring; (c) a pre-mix dispersion comprising fluoropolymer nanoparticles particles having a mean particle size of from about 10 nm and about 500 nm and a surfactant; and (d) an auxiliary surfactant comprising a cationic fluorosurfactant, wherein the composition has a pH of from about 8 to about
 14. 2. The electrolytic plating composition of claim 1 wherein the auxiliary surfactant further comprises an anionic fluorosurfactant.
 3. The electrolytic plating composition of claim 1 wherein the electrolytic plating composition is essentially free of any nonionic fluorosurfactants.
 4. (canceled)
 5. The electrolytic plating composition of claim 2 wherein the cationic fluorosurfactant comprises a fluoroalkyl ammonium halide surfactant.
 6. The electrolytic plating composition of claim 1 wherein the fluoropolymer nanoparticles have a specific surface area between about 15 m²/g and about 50 m²/g.
 7. (canceled)
 8. The electrolytic plating composition of claim 1 wherein the electrolytic plating composition comprises about one gram of cationic fluorosurfactant for every about 20 m² to about 80 m² of surface area of fluoropolymer particle. 9.-19. (canceled)
 20. The electrolytic plating composition of claim 1 wherein the compound comprising the nitrogen-containing heterocyclic ring is selected from the group consisting of substituted and unsubstituted hydantoin and substituted and unsubstituted succinimide.
 21. The electrolytic plating composition of claim 1 wherein the complexing agent comprises succinimide.
 22. The electrolytic plating composition of claim 1 wherein the complexing agent comprises a hydantoin compound of formula I

wherein R¹, R², R³, and R⁴ are independently hydrogen, an alkyl group having 1 to 5 carbon atoms, a hydroxyalkyl group having 1 to 5 carbon atoms, or a substituted or unsubstituted aryl group. 23.-36. (canceled)
 37. The electrolytic plating composition of claim 1 wherein: the auxiliary surfactant further comprises an anionic fluorosurfactant; the electrolytic plating composition is essentially free of any nonionic fluorosurfactants; the cationic fluorosurfactant comprises a fluoroalkyl ammonium halide surfactant in a concentration from about 0.5 g/L to about 10 g/L and about one gram of cationic fluorosurfactant for every about 10 m² to about 80 m² surface area of fluoropolymer particle; the fluoropolymer nanoparticles have a specific surface area between about 15 m²/g and about 50 m²/g, a mean particle size of from about 10 nm to about 200 and a concentration of between about 2 g/L and 200 g/L; the silver ion concentration is from about 10 g/L to about 100 g/L; the compound comprising the nitrogen-containing heterocyclic ring is selected from the group consisting of substituted and unsubstituted hydantoin and substituted and unsubstituted succinimide; the electrolytic plating composition further comprises a wetting agent comprising sulfopropylated polyalkoxylated beta-naphthol, condensation products of naphthalenesulfonic acid, or salts thereof; the electrolytic plating composition further comprises a sulfonic acid and/or a derivative of a sulfonic acid at a concentration between 50 g/L and 500 g/L; the electrolytic plating composition further comprises about 30 mg/L to about 500 mg/L of potassium bromide; the electrolytic plating composition has a pH from about 9.5 to about 11.5, or from about 9 to about
 11. 38. A process for applying a silver or silver alloy composite coating onto a substrate surface, the process comprising: contacting the substrate surface with an electrolytic plating composition of claim 1 and applying an external source of electrons to the electrolytic plating composition to thereby electrolytically deposit the composite coating onto the substrate surface, wherein the composite coating comprises silver or silver alloy and the fluoropolymer nanoparticles.
 39. The process of claim 38 wherein the composite coating has a coefficient of friction, from about 0.05 to about 0.3.
 40. The process of claim 38 or 39 wherein the composite coating contains at least 4.0 wt. % of fluorine (atomic).
 41. (canceled)
 42. The process of claim 40 wherein the composite coating has a contact resistance less than about 5 milliohms. 43.-47. (canceled)
 48. The process of claim 42 wherein the composite coating has a thickness from about 1 μm to about 10 μm.
 49. (canceled)
 50. A process for preparing an electrolytic plating composition, the process comprising: (1) mixing a complexing agent comprising a hydantoin compound, any wetting agent and conductive salt, and water to form a solution; (2) mixing a silver ion source comprising Ag-MSA in the solution; (3) mixing an auxiliary surfactant and any alkali bromide in the solution; (4) following steps (1)-(3), mixing a pre-mix dispersion comprising fluoropolymer nanoparticles particles having a mean particle size of from about 10 nm and about 500 nm and a surfactant along with any antifoam additive in the solution to form the electrolytic plating composition; and (5) optionally maintaining the pH of the mixture at about 9 to about 10 by addition of base. 51.-52. (canceled) 